Fuel cells for combining hydrogen and oxygen to produce electricity are well known. A known class of fuel cells includes a solid-oxide electrolyte layer through which oxygen anions migrate; such fuel cells are referred to in the art as “solid-oxide” fuel cells (SOFCs).
In some applications, for example, as an auxiliary power unit (APU) for an automotive vehicle, an SOFC is preferably fueled by “reformate” gas, which is the effluent from a catalytic liquid hydrocarbon oxidizing reformer, also referred to herein as “fuel gas”. Reformate typically includes amounts of carbon monoxide (CO) as fuel in addition to molecular hydrogen. The reforming operation and the fuel cell operation may be considered as first and second oxidative steps of the liquid hydrocarbon, resulting ultimately in water and carbon dioxide. Both reactions are exothermic, and both are preferably carried out at relatively high temperatures, for example, in the range of 700° C. to 1000° C.
A complete fuel cell stack assembly includes fuel cell subassemblies and a plurality of components known in the art as interconnects, which electrically connect the individual fuel cell subassemblies in series. Typically, the interconnects include a conductive foam or weave disposed in the fuel gas and air flow spaces adjacent the anodes and cathodes of the subassemblies.
In the prior art, a fuel cell stack is assembled typically by laying up the interconnects and the fuel cell subassemblies in a jig, forming repetitive fuel cell units. Typically, a fuel cell subassembly comprises a cathode layer coated onto a middle ceramic solid-oxide electrolyte layer which, in turn, is attached to a relatively thick, structurally-significant anode element. In such a prior art assembly, each of the elements in the stack, including the fuel cell subassemblies, becomes a structural and load-bearing element of the stack.
In our commonly assigned, co-pending application Ser. No. 11/027,095, the entire disclosure of which is incorporated herein by reference, a novel modular fuel cell cassette is disclosed wherein a plurality of individually assembled cassettes are assembled into a fuel cell stack. Each cassette basically includes a mounting plate having a main opening wherein the fuel cell subassembly component is mounted, a separator plate and an interconnect. The mounting and separator plates have two sets of registered, alternating openings spaced about their perimeters defining fuel gas and air passages, respectively. The components are sealed at various, strategic locations to create the air-tight and separate channels for proper movement of the fuel gas and air through the anode and cathode spaces, respectively. In this regard, it is understood by those skilled in the art that the fuel gas and air channels should not be allowed to mix. Since each cassette is individually assembled and sealed prior to assembly in the stack, each cassette can be tested for leaks prior to being added to the fuel cell stack, a major improvement over the prior art where leaks were discovered only after full stack assembly.
The assembly, and particularly the sealing, of the fuel cell components presents a unique challenge in that all materials must function, bond and hold a seal at temperatures of 700-800 degrees centigrade. Previously, the fuel cells themselves and the cassettes they are mounted in, were joined and sealed with a glass bonding and sealing system. During sintering, the glass devitrifies and flows to fill the interface between the components to bond and seal them. This seal functioned satisfactorily until the stack is thermal cycled. Due to differences in the coefficient of thermal expansion between components and glass seal, and the fact that the glass progressively crystallizes, the seal fractures resulting in leakage. As the leak progressively increases cell output degrades until the voltage is so low that the output is not usable. In the glass sealing system the space between the surfaces to be bonded and or sealed is maintained by Yttrium Stabilized Zirconium (YSZ) beads.
To overcome the issues experienced with glass, brazing alloys have been developed to bond to the materials and to operate at the necessary temperatures. The issue experienced with braze is if the surfaces are precisely flat and clamped together the braze alloy may be squeezed out of the joint interface. This results in a very weak joint and displaced metal may migrate onto adjacent areas causing a short circuit of the fuel cell or other damage to the cathode. In addition, if the parts are not flat, braze material will be squeezed out of the minimal gap areas. In either situation, the joint is not hermetically sealed resulting in joint leakage and degradation.
Previously, braze space was created by mixing particles of material of 30 to 50 microns into the alloy paste that had a melt temperature greater than 980 degrees C. While the addition of particles for creating braze space was generally functional, it was found that the particles consumed some of the copper oxide (the component of the braze required to bond to ceramic) reducing the effectiveness of the joining system.
There therefore remains a need in the art for creating a joint seal between components of a fuel cell assembly wherein the seal maintains its integrity through all intended manufacturing steps, as well as during the expected usable life of, the fuel cell.